1. Field of the Invention
The present invention relates to optical systems including a laser device having an operational wavelength, and in particular to arrangements for locking the operational wavelength to a desired frequency. Such laser devices include semiconductor laser diodes.
2. The Background Art
Semiconductor laser diodes generally use a grating fabricated within the laser cavity to determine the operational wavelength, i.e. the wavelength at which laser operation occurs. There are several causes of drift in the operating wavelength, for example laser chip aging, thermistor aging or thermoelectric control electronics aging. Typically, when operated at constant temperature, the internal grating prevents the laser wavelength from drifting more than 0.1 nm over the course of the lifetime of the semiconductor laser diode. For some applications, such as CATV using Dense Wavelength Division Multiplexing (DWDM), this level of drift is not acceptable.
If an internal grating does not provide sufficient wavelength stability, then some form of wavelength locker is normally utilised. A wavelength locker monitors the laser wavelength and adjusts the operating temperature of the semiconductor laser diode so as to control the stability of the laser wavelength, typically to within about 0.02 nm.
FIGS. 1 and 2 show a typical implementation of wavelength locking in a laser module 1, which may be a standard 14-pin butterfly package, as discussed in the article “Wavelength Monitor Integrated CW DFB Laser Module for DWDM Applications” by Nasu et al. in the Furukawa Review, No. 23, pp 6-10 (2003). As shown in FIG. 1, the laser module 1 has a distributed feedback (DFB) laser diode 3 which outputs laser beams from both a front facet and a rear facet. The power of the laser beam output from the front facet, which forms the output of the laser module, is much higher that that output from the rear facet.
The laser beam output from the front facet of the laser diode 3 passes though a front collimating lens 5, an isolator 7 and then an objective lens 9 that couples the laser beam into an optical fiber 11. The laser beam output from the rear facet of the laser diode 3 is directed to a wavelength locking arrangement. In particular, the laser beam output from the rear facet of the laser diode 3 passes through a rear collimating lens 13 to produce a collimated light beam, and then a prism 15 which splits the collimated light beam into two diverging collimated light beams. One of the diverging collimated light beams is directly incident onto a first monitor photodiode 17 to generate a wavelength independent signal, while the other of the diverging collimated light beams is directed through an etalon 19, matched to a required operational wavelength, and subsequently onto a second monitor photodiode 21. As the light beam incident on the etalon 19 is collimated, the etalon 19 effectively acts as a narrow-band filter centred at the required operational wavelength, and accordingly the second monitor photodiode 21 generates a wavelength dependent signal.
As shown in FIG. 2, the components illustrated in FIG. 1 are mounted onto a support substrate 23, which is in turn mounted onto a thermoelectric cooler 25 which uses the Peltier effect to vary the temperature of the DFB laser diode 3 in order to vary the operational wavelength of the DFB laser diode. A processor (not shown) processes the wavelength independent signal and the wavelength dependent signal respectively produced by the first monitor photodiode 17 and the second monitor photodiode 21 to generate a control signal for the thermoelectric cooler 25 which varies the temperature of the DFB laser diode 3 to lock the operational wavelength to the required operational wavelength.
The collimating optics in the wavelength locking arrangement of FIGS. 1 and 2 increase the size of the device. U.S. Pat. No. 5,825,792 proposes a compact wavelength locker is which the laser beam output from the rear facet of a DFB laser diode passes uncollimated though an etalon. As the laser beam is uncollimated, the transmission properties of the etalon vary with the angle at which the laser beam passes through the etalon. Light passing through the etalon is directed onto two closely-spaced photodiodes. The differential output of the two photodiodes is used in a feedback loop to stabilize the operational wavelength of the DFB laser diode. This arrangement relies on the output of the two photodiodes being the same when the operational wavelength of the DFB laser diode is the required wavelength, which involves fine adjustment of the alignment and the responses of the two photodiodes.
An object of the present invention is to provide an alternative wavelength locking arrangement which is both compact and low cost.